The present invention relates to an apparatus for classifying dust materials in a suction pneumatic conveying system operating at pressures substantially between 0.6 and 0.8 bar, comprising a collecting tank having connected thereto a telescoping exhaust pipe. Separators and a pneumatic suction generator are arranged downstream of the collecting tank for the granulometrically classified separation of hot highly abrasive filling powder such as is exhausted particularly from furnaces for the production of block electrodes for aluminium fusion electrolysis after completion of the electrodes.
Said electrode production is described by LUEGER, "Lexikon der Technik", RORO Edition, Vol. 47, pages 164 to 166. For such production, the prepressed block electrodes are introduced, in their green state, into low furnaces, so-called pit furnaces, embedded in filling powder and heated slowly to 1300.degree. C. to 1400.degree. C. and slowly cooled again. This process normally extends over a period of several weeks.
The filling powder generally has a coking coal basis. With a view to an optimally low-dust embedding and obtaining of an optimal granulometry of the filling powder to permit uniform degassing, it is generally necessary to segregate the ultrafine dusts from the reusable filling powder and separate them into granulometric fractions.
For this purpose, heretofore, the filling powder has been sucked out of the stationary pit furnaces into a container mounted on a travelling gantry crane, which was then transported to the end of the furnace shed, let off there and the material was separated into the required granulometric fractions in a stationary screening plant.
This method is on the one hand rather complicated, and on the other hand the space conditions on the crane trolly, the high operating temperature, the low negative pressure and the high throughput capacities do not permit the direct and additional installation of a screening machine on the crane structure.
Air classification is known as a further method of separating powder type bulk materials. This is generally performed in a low pressure zone. For this purpose a segregation and locking out of the vacuum zone would be necessary after the exhaustion of the filling powder. This is impossible in view of the restricted height conditions which already exist in the furnace sheds.